ORA-20000

Error:

SQL> conn / as sysdba
Connected to an idle instance.
SQL> shutdown immediate;
ORA-01034: ORACLE not available
ORA-27102: out of memory
Linux-x86_64 Error: 12: Cannot allocate memory
Additional information: 1
Additional information: 3407880
Additional information: 8
SQL> startup;
ORA-00845: MEMORY_TARGET not supported on this system

Cause: The MEMORY_TARGET parameter was not supported on this operating system or /dev/shm was not sized correctly on Linux(the size of /dev/shm needs to be greater than MEMORY_TARGET or MEMMORY_MAX_TARGET).

Action: Check the size of /dev/shm and increase the size of /dev/shm

Check /dev/shm:

[root]# df -h /dev/shm
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 2.0G  1.2G  0.8G  60% /dev/shm

Change your system file /etc/fstab:

[root]# vi /etc/fstab
tmpfs  /dev/shm  tmpfs  defaults,size=3G  0 0

And remount:

[root]# mount -o remount /dev/shm

Chech changes:

[root]# df -h /dev/shm
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 3.0G  1.2G  1.9G  40% /dev/shm

Startup database:

SQL> conn / as sysdba;
Connected to an idle instance.
SQL> startup;
ORACLE instance started.

Total System Global Area 1937457152 bytes
Fixed Size                  2214576 bytes
Variable Size            1476396368 bytes
Database Buffers          452984832 bytes
Redo Buffers                5861376 bytes
Database mounted.
Database opened.
SQL>